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November 22, 2016

7.4 quake was triggered by vertical split in undersea rock: experts


Staff Writers

The major earthquake that triggered tsunami in the Tohoku region Tuesday was caused by a type of jolt that is likely to cause tidal waves, according to the Meteorological Agency.

The temblor also caused tsunami across a widespread area because it originated at a shallow point under the seabed.

The tsunami measuring up to 1.4 meters were the highest since the magnitude-9 Great East Japan Earthquake of March 11, 2011, that killed more than 15,000 people.

The magnitude-7.4 quake Tuesday jolted northeastern Japan early at 5:59 a.m. in a vertical split of an undersea rock.

In this mechanism, a rock plate 60 km off Iwaki, Fukushima Prefecture slid vertically, creating a gap in the seabed level and pushing the seawater up, seismology experts said.

The Meteorological Agency said it considers the quake an aftershock of the 2011 earthquake. Experts said people need to be alert for further aftershocks for the next few days.

“Although I personally don’t think there will be a massive magnitude-9 earthquake, there is possibility of a similar scale of earthquake,” said Masanobu Shishikura of the National Institute of Advanced Industrial Science and Technology (AIST).

Shinji Toda, a professor at Tohoku University’s International Research Institute of Disaster Science, agreed, saying it is possible another magnitude-7 quake could hit the Tohoku region.

Meanwhile, Manabu Takahashi, a professor at the Institute of Disaster Mitigation for Urban Cultural Heritage at Ritsumeikan University, said even though the magnitude-7.4 quake was relatively strong, it was not surprising.

“Looking at data accumulated over the past 100 years, earthquakes with magnitudes of around 7 occur about three times in five years” in Japan, Takahashi said. “As it caused a tsunami, it’s fair to say it was a moderately large quake, but not a major quake.”

He also said there could be a larger earthquake in the near future.

“The Meteorological Agency has previously considered a first big quake as the main shock, but you’ll never know when the biggest one will come,” Takahashi said.

A series of magnitude-7 earthquakes have occurred in the Pacific Ocean off Japan since seismic activities intensified after the 2011 earthquake, which is estimated to have had 200 times more energy than Tuesday’s temblor.

“Before March 11, we didn’t see much of this type of earthquake in the area. But since the major calamity, we’ve started to see it in the region,” Shishikura of AIST said.

In that sense, the quake could have been triggered by the March 11 earthquake, he said.

Information from Kyodo added

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